As one information reading apparatus of this type, there is a magnetic sensing apparatus having a line sensor with a plurality of magnetic sensors whose resistance values change when a magnetic field is applied, and which are disposed in a line. The magnetic sensing apparatus is used in, e.g., a paper currency identification apparatus that identifies paper currency. By means of the magnetic sensors, the paper currency identification apparatus detects an image printed with magnetic ink on the paper currency to be identified. The paper currency identification apparatus also makes determination on the validity, denomination, and so on of the paper currency on the basis of similarity between the detected printed image and a stored reference image.
Because the magnetic field due to the magnetic ink used to print the image on the paper currency or the like is quite weak, the sensing signal obtained by the magnetic sensing apparatus is quite weak. Moreover, a direct current component (offset component) is added to the sensing signal in the magnetic sensing apparatus. The sensing signal must accordingly be amplified and the offset component must be removed. The offset component varies, however, with environmental changes such as changes in the ambient temperature, and with aging changes and the like in the magnetic sensors. It is therefore not easy to separate the offset component in the sensing signal.
A technique for correcting non-uniformity in the level of the sensing signal due to the temperature of the magnetic sensing elements and so on is disclosed in the magnetic pattern sensing apparatus described in patent reference 1. The magnetic pattern sensing apparatus in patent reference 1 performs an offset adjustment during each interval between the passage of one piece of paper currency through the magnetic sensing apparatus and the passage of the next piece of paper currency through the magnetic sensing apparatus.